quick question: positrack rear end?

You can do this by changing the differential settings as follows:

Set "type" to "lsd", set "state" to "closed", and set "closedTorque" to some reasonable value, like 100-200 (the Grand Marshal uses 150 for its limited slip differential).

You could really just copy the differentials line or section from the Grand Marshal's LSD ("fullsize_differential_R_LSD" in "fullsize_suspension_R.jbeam").

We don't have any more advanced LSD options than this right now. The higher the closedTorque value, the more locked up the differential is. If you set it to something really high like 10000 you'll basically have a locked/welded diff. 0 would be an open diff.